The present invention relates to a rolling mill including a mechanism for controlling the flatness of the rolled product.
In order to control the sectional flatness of the rolled product across its width direction, various methods for bending the work rolls in a four high rolling mill have been developed in recent years. However, such methods adapted hitherto are found to have a limited capability of controlling the flatness of the rolled product. Especially when the width of stocks or workpieces varies widely, the satisfactory control cannot be accomplished.
In order to obtain a rolled product with a uniform flatness and especially having a small variation of thickness across its width, it is important to reduce deformation of the work roll due to the rolling load as far as practicable and, at the same time, increase the capacity for correction by roll bending. However, in the conventional rolling mills, the ends of a work roll receive the bending moment due to the load owing to the contact with a backup roll. As a result, the portions adjacent to the edges of the rolled product are considerably deformed. Especially when the width of a stock or workpiece changes, the above-described bending moment also varies. The narrower the width of the stock, the work rolls receive greater bending moments so that they are considerably deflected, resulting in thickness variation over a wide range. Furthermore, the work rolls are thermally expanded and such thermal expansions are not uniform in the axial direction so that a thermal crown results. In addition, due to the wear of the work rolls, the initial roll crown is varied.
In order to overcome the above and other problems, there must be provided a powerful roll bending device so that the roll crown may be freely varied. However, in the conventional rolling mill, the work rolls are restrained by the backup rolls with a high degree of bending rigidity so that the roll bending effects are exerted only at the portions adjacent to the ends of the work rolls. Furthermore, the roll bending force is limited by the rigidity of the roll necks and the life of bearings. As a result, the roll bending force is limited so that the roll crown cannot be varied as desired. Thus it has been impossible to overcome the above and other problems. As a result, with the conventional rolling mills, whenever the thickness of the stock is varied, the roll crown must be varied accordingly. Therefore the work rolls must be replaced very frequently so that the operation efficiency is considerably decreased. In addition, many work rolls with various crowns must be provided so that the costs are increased.
One of the objects of the present invention is therefore to provide a rolling mill with a considerably improved capability of controlling the flatness of the rolled product.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.